Abstract: A typical nuclear well logging tool forms a count rate data stream including statistical noise. A robust single pass partially deconvolved smoothing filter matched to the tool response is applied to the count rate data to enhance vertical resolution while minimizing the statistical noise increase. The smoothing filter is partially deconvolved by the application of a three point deconvolution characteristic of the cusp portion of a related cusp function and a box function representative of the tool response.

Abstract: A method for providing gain stabilization to a nuclear spectroscopy tool by use of gross features of largely invariant spectra. A particle or gamma ray energy spectrum of an earth formation and wellbore are measured. A ratio of the number of particles or gamma rays measured in a first energy window to the number of particles or gamma rays measured in a second energy window is calculated. The gain of the nuclear spectroscopy tool is adjusted as a function of the calculated ratio. The first energy window is selected to span substantially an entire range of measured energies for the spectrum and the second energy window is selected to span an upper range of the measured energies for the spectrum.

Abstract: A system for measuring fast neutron induced gamma ray energy spectra in cased wellbores. The system includes an electronic source of monoenergetic 14 MeV fast neutrons, a high density bismuth germanate scintillation detector, a downhole pulse height analyzer and a highly accurate gain stabilizer circuit. The gain stabilization utilizes the iron (Fe) edge in the catpure gamma ray spectrum to establish system gain since iron is always present in the tool case and the wellbore casing.

Abstract: An efficient spatial smoothing algorithm for filtering data while preserving spatial detail is obtained using the system (impulse response) function of the sensor. In contrast to the normal procedure for determining the filter coefficients for an arbitrary system function, this technique avoids the use of Fourier transforms. The spatial filtering coefficients are obtained analytically for the frequently applicable Gaussian system function. The efficiency of this procedure is illustrated by filtering discrete data. For realtime smoothing of nuclear log data, a filter length of five times the vertical resolution is required.

Abstract: Methods for measuring thermal neutron decay characteristics of earth formations include the steps of irradiating an earth formation with a discrete burst of fast neutrons, detecting indications of thermal neutron concentration in the formation during a sequence of discrete time gates, determining the zeroth order moment of the indications during the sequence of discrete time gates, and determining the first order moment of the indications during the sequence of discrete time gates, in order to obtain the desired thermal neutron decay characteristic of the earth formation.

Abstract: In the exemplary embodiments of the invention disclosed, the decay of the thermal neutron concentration in an earth formation following irradiation thereof with a burst of fast neutrons is measured by detecting indications of the thermal neutron concentration during a sequence of contiguous, discrete time gates between neutron bursts. The time gate sequence begins following a discrete time delay after the commencement of the preceding neutron burst and extends over the entire, or substantially the entire, remainder of the interval between neutron bursts. The time gates are grouped into a number of gate groups, e.g. four, each of which includes a number, e.g. four, of discrete time gates. Both the time gates within each group and the groups themselves are contiguous in time. The durations of the individual discrete time gates within the separate groups are equal, but the duration of the time gates progressively increases from group to group in the sequence.